1. Field of the Invention
The present invention relates to a method for polishing and chamfering a rare earth alloy, and a method and machine for sorting out polishing media. More particularly, the present invention relates to a polishing and chamfering method using polishing media, like a barrel polishing method, a method for sorting out polishing media used for polishing from objects to be polished, and a machine used for the sorting of the polishing media.
2. Description of Related Art
Rare earth alloys have been used as materials for strong magnets. Rare earth magnets produced by magnetizing rare earth alloys have been suitably used as a material for a voice coil motor (VCM) for positioning a magnetic head of a magnetic recording apparatus, for example.
Barrel polishing has been conventionally employed due to its superiority in mass-productivity for polishing and chamfering work pieces of a rare earth alloy. As used herein, chamfering refers to polishing a work piece to round the edges of the work piece. Two types of barrel polishing, rotary barrel polishing and vibration barrel polishing, are available. Rotary barrel polishing is widely used because a machine for this polishing is inexpensively available.
Rare earth alloys are brittle materials and are therefore easily chipped. To avoid chipping, it is generally suitable to adopt vibration barrel polishing rather than rotary barrel polishing. The reason is presumed to be that in vibration barrel polishing, a spiral flowing state is established, and this reduces collision between polishing media (hereinafter referred to as xe2x80x9cmediaxe2x80x9d) and objects to be polished and also between objects to be polished. Media and objects to be polished are rubbed against each other while flowing spirally in the same direction. In this way, polishing proceeds moderately.
Japanese Laid-Open Patent Publication No. 5-208360 discloses a vibration barrel polishing method in which a spiral flow is generated in a liquid in which all of the media and objects to be polished are immersed when a barrel bath is in the horizontal position, and describes that chipping can be further suppressed by this method.
The present inventors examined in various ways the methods for chamfering a rare earth alloy and found that, although the above conventional vibration barrel polishing method can suppress chipping to some extent, it may fail to provide uniform chamfering. In particular, this failure in uniform chamfering occurs when a plurality of flat work pieces are to be polished at one time, for example. In such a case, the work pieces firmly adhere to each other in the barrel bath, and thus part of edges of the work pieces are kept away from contact with the media.
In addition, chipping may occur even using the vibration barrel polishing method when the object to be polished is a work piece of a rare earth alloy including a rigid major phase mainly causing a brittle fracture and a grain boundary phase causing a ductile fracture, such as a rare earth alloy produced by sintering (hereinafter, referred to as a xe2x80x9crare earth sintered alloyxe2x80x9d).
Chipping may also occur in a process of sorting out the media from the work pieces (objects to be polished) after the barrel polishing process, in which work pieces tend to collide against each other. As the sorting method, generally used is a method (called a xe2x80x9csifting methodxe2x80x9d) in which a mixture of media and work pieces is put in a sieve and the media are selectively dropped through the sieve under vibration. In this sifting method, the work pieces left on the sieve inevitably collide against each other, and thus chipping occurs.
In addition, the work pieces made to firmly adhere to each other in the barrel polishing process as described above are kept in the adhesion state in the sorting process. This means that a liquid medium (typically, water) with which the work pieces firmly adhere to each other is kept in touch with the work pieces for a comparatively long time. The rare earth alloy described above, which is highly susceptible to corrosion (easily rusts), should preferably be swiftly cleaned and surface-treated after chamfering. However, swift execution of these processes is difficult for such work pieces that firmly adhere to each other, and thus corrosion tends to occur.
An object of the present invention is to provide a method, in which polishing media is used to polish work pieces like a barrel polishing method, for chamfering a rare earth alloy that can suppress chipping of work pieces and also suppress adhesion of work pieces to each other during polishing to enable uniform chamfering.
Another object of the present invention is to provide a method for sorting out ball media while suppressing collisions between objects to be polished, and a machine for sorting out ball media permitting such sorting.
The method for chamfering a rare earth alloy of the present invention includes the following steps: preparing rare earth alloy work pieces, feeding the rare earth alloy work pieces, media, a liquid medium, and spacer particles (0.1% to 10% by volume of the liquid medium) into a container, and subjecting the rare earth work to polishing by vibrating the container.
The liquid medium is preferably fed into the container so that at least three-fourths of the bulk volume of the rare earth alloy work and the media are immersed in the liquid medium.
The average particle size of the spacer particles is preferably in a range of 0.05 mm to 1 mm, more preferably in a range of 0.1 mm to 0.5 mm.
The specific gravity of the media and the specific gravity of the spacer particles are preferably 4 or less.
The media preferably include alumina grains and a binder, and the weight percentage of the alumina grains is in a range of 45% to 48%. The porosity of the media is preferably 3% or less.
In the step of subjecting the rare earth work pieces to polishing, the container is preferably vibrated so that the amplitude in the horizontal direction is 0.4 mm or more and the amplitude in the vertical direction is 0.15 mm or more. Also, the container is preferably vibrated at an acceleration of 800 mm/sec2 or more in the horizontal direction.
Ball media is preferably used as the media.
The method for sorting out ball media of the present invention includes supplying a mixture including ball media and an object to be polished to a slope, and allowing the ball media to roll along the slope to be removed from the slope, and the object to be polished to stay on the slope.
The step of supplying a mixture preferably includes the step of moving the slope. More preferably, the slope includes a ring belt having an opening in the center, and the slope is moved by rotating the ring belt.
The method may further include the step of collecting the ball media dropping from the opening formed in the center of the ring belt.
The machine for sorting out ball media of the present invention includes a slope including a ring belt having an opening in the center, for receiving a mixture including ball media and an object to be polished, and a driver for rotating the slope.
The slope preferably includes a rubber layer on the surface.
Hereinafter, the function of the present invention will be described.
The method for chamfering a rare earth alloy of the present invention, which adopts a polishing method utilizing vibration of a mixture of polishing media and work pieces, like a vibration barrel polishing method, can suppress chipping of work pieces of a highly brittle rare earth alloy. In particular, by using balls as the media, chipping can be further effectively suppressed. Especially, ball media are preferably used for work pieces including a rigid major phase mainly causing brittle fracture and a grain boundary phase causing a ductile fracture, such as work pieces of a rare earth alloy produced by sintering (rare earth sintered alloy), which are especially easily chipped. Moreover, using ball media makes it possible to employ a method for sorting out ball media described later. By employing this method, chipping in the sorting process can be suppressed.
The spacer particles (0.1% to 10% by volume of the liquid medium), supplied together with the rare earth alloy work pieces and the media, attach to the surfaces of the rare earth alloy work pieces, and serve to prevent the work pieces from adhering to each other due to surface tension of the liquid medium during polishing. If the volume content of the spacer particles is less than 0.1%, the effect of suppressing adhesion of the work pieces to each other is not sufficient. If it exceeds 10%, the polishing efficiency may be reduced. The content of the spacer particles is more preferably in the range of 0.3% to 3% by volume of the liquid medium from the standpoints of the adhesion prevention effect and the polishing efficiency. In particular, in the case of using ball media, the inter-medium space tends to be large, compared with the case of using triangle-shaped media (including media in a triangular pyramid shape and cone-shaped media), and thus work pieces easily adhere to each other. In this case, therefore, the effect obtained by adding the spacer particles is especially great.
The liquid medium is fed into the container so that at least three-fourths of the rare earth alloy work pieces and the media put in the container are immersed in the liquid medium. By this immersion, the adhesion of the work pieces to each other can be further effectively suppressed. More preferably, the liquid medium is fed so that the entirety of the rare earth alloy work pieces and the media are immersed in the liquid medium. A liquid medium prepared by adding an anticorrosive and a surfactant to water is suitable for use. The anticorrosive is preferably added because a rare earth alloy easily corrodes (rusts).
If the rare earth alloy work pieces adhere to each other, some edges thereof fail to collide with the media, resulting in non-uniform chamfering. According to the method of the present invention, since such adhesion of work pieces to each other is suppressed, uniform chamfering is realized. In addition, in the sorting process after the polishing, the rare earth alloy work pieces can be easily collected as independent pieces from each other since they are free from adhering to each other. Also, having no adhesion of work pieces to each other, the liquid medium is not allowed to stay between the work pieces. This provides the effect of suppressing corrosion of the rare earth alloy work pieces.
The xe2x80x9cspacer particlesxe2x80x9d as used herein refers to particles having the function of forming spaces between the rare earth alloy work pieces as the objects to be polished (suppressing adhesion of the work pieces to each other) as described above. Various particles can be used as the spacer particles. The average particle size of the spacer particles is preferably in the range of 0.05 mm to 1 mm, more preferably in the range of 0.1 mm to 0.5 mm. The shape of the particles is preferably closer to a sphere. As such spacer particles, polishing particles may be used, but polishing ability is not necessarily required. For example, polymer particles may be used.
The specific gravity of the media is preferably 4 or less to enable suppression of chipping of rare earth alloy work pieces. The media having a specific gravity exceeding 4 gives great impact to the rare earth alloy work pieces, causing chipping. Therefore, it is preferable to use media having a specific gravity of 4 or less. As for the spacer particles, particles having a specific gravity of 4 or less are easily dispersed in the liquid medium uniformly by the movement of the media and the liquid medium, and thus are preferred from the standpoint of preventing adhesion of work pieces to each other.
As the media, it is preferable to use media including alumina grains and a binder where the weight percentage of the alumina grains is in the range of about 45% to about 48%. By using such media, appropriate polishing efficiency is obtained while chipping of the rare earth alloy work pieces is suppressed. Moreover, by using media having a porosity of 3% or less, appropriate polishing efficiency is obtained while chipping of the work pieces is suppressed.
In the vibration polishing process, the container is vibrated so that the amplitude in the horizontal direction exceeds 0.4 mm. By this vibration, the polishing efficiency can be enhanced while chipping is suppressed. During this vibration, the amplitude in the vertical direction is preferably 0.15 mm or more. Moreover, the polishing efficiency can be further enhanced by vibrating the container at an acceleration of 800 mm/sec2 or more in the horizontal direction.
In the method for sorting out the ball media of the present invention, the mixture including the ball media and the objects to be polished is supplied to the slope. The ball media, which are spheres, roll along the slope to be finally removed from the slope. The objects to be polished, which are generally polyhedrons having edges and therefore have high friction resistance against the slope, stay on the slope. In other words, the ball media and the objects to be polished are sorted from each other by the fact that the ball media having a easy-to-roll shape are removed from the slope on one hand and the objects to be polished having a shape causing large friction against the slope stay on the slope on the other hand.
By moving the slope to which the mixture including the ball media and the objects to be polished are supplied, the sorting of the ball media from the objects to be polished can be made efficiently even when the mixture is supplied continuously. More specifically, if the area of the slope is too small for the amount of the mixture supplied, the objects to be polished stay on the slope at a high density, blocking the rolling of the ball media and causing chipping of the objects due to collision between the objects. By moving the slope, that is, by supplying the mixture continuously to portions of the slope on which no objects stay, the above problems can be prevented.
Continuous movement of the slope can be realized by a comparatively simple construction by forming a ring belt having an opening in the center as the slope and rotating the ring belt continuously. More specifically, the mixture is supplied to a portion of the slope, and the slope on which the objects to be polished selectively stay is rotated. The objects staying on the portion of the slope are collected by the time when the portion of the slope returns to the position at which the mixture is supplied. The ball media dropping from the opening formed in the center of the ring belt are collected in a container. Thus, the ball media can be easily collected.
The machine for sorting out ball media of the present invention includes the slope formed of a ring belt having an opening in the center for receiving the mixture including the ball media and the objects to be polished and a driver for rotating the slope. With this machine, the sorting method described above can be efficiently executed. Moreover, by forming a rubber layer on the surface of the slope, even objects to be polished in a comparatively easy-to-roll shape can be kept to stay on the slope stably and reliably.